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The inch, the foot, the yard and the mile are all U.S. customary units of length derived from the imperial system. This system of measurement is used primarily in the United States. The inch is accepted internationally as the equivalent of 2.54 centimeters and 25.4 millimeters. The use of the inch as a unit of measurement in Great Britain dates back at least to the seventh century. In 1066, the length of three dried barleycorns stretched end to end defined the length of the inch. The word "inch" comes from the Latin word "uncia," which means 1/12 part.